1. Field of the invention
The present invention relates to an ultrasonic transmitting and receiving apparatus to be used for obtaining ultrasonic images by transmitting ultrasonic waves and receiving ultrasonic echoes.
2. Description of a Related Art
FIG. 21 shows the structure of ultrasonic transducers included in an ultrasonic probe that is generally used in a conventional ultrasonic transmitting and receiving apparatus, and acoustic field distribution of an ultrasonic beam transmitted from the transducers. As shown in FIG. 21, the ultrasonic transducer array 100 is fabricated, for example, by linearly arranging a large number of piezoelectric elements 101 having electrodes 102 and 103 formed on both ends thereof. Drive signal generating circuits including pulsers etc. are connected to the electrodes 102 and 103. Applying a voltage to the electrodes provided on the piezoelectric element, the piezoelectric element expands and contracts by piezoelectric effect to generate ultrasonic waves. By driving plural piezoelectric elements at predetermined time intervals, spherical waves transmitted from the respective piezoelectric elements are synthesized and a focal point F of an ultrasonic beam can be formed in a desired direction and a desired depth.
Thus formed acoustic field of the ultrasonic beam is defined by an angle 106  when seeing the position of the focal point from the aperture of the ultrasonic transducer array 100 and a directivity angle Θ determined by the aperture of the transducer. In an ultrasonic beam formed by the phase delay method, the directivity angle Θ is an angle formed by a region, where beam intensity becomes zero by the synthesis of plural ultrasonic waves, and the X axis, and it represents the spread of the ultrasonic beam.
As shown in FIG. 21, the ultrasonic waves transmitted from the ultrasonic transducer array 100 are converged in the vicinity of the focal point F and diffused again. That is, the beam diameter of the ultrasonic beam differs according to the distance (depth) from the ultrasonic transducer array 100.
By the way, image quality of an ultrasonic image largely depends on the acoustic pressure intensity and beam diameter of transmitted and received ultrasonic beams. Since strong signal intensity is obtained by using an ultrasonic beam having high acoustic pressure intensity, slight change of the medium within an object to be inspected can be detected. Further, by using an ultrasonic beam having a narrow beam diameter, spatially detailed ultrasonic image information can be obtained. It is desired that the acoustic pressure intensity and the beam diameter are not only satisfactory values naturally, but also uniform over the imaging region. Because, if these values vary, the image quality of the ultrasonic image becomes nonuniform within the image to interfere with satisfactory ultrasonic diagnosis.
The longer the propagation distance, the more the ultrasonic wave is attenuated. Accordingly, the deeper the region of the object where an ultrasonic wave is reflected and an ultrasonic echo is generated, the weaker the detection signal of the ultrasonic echo. Therefore, in order to correct such attenuation of the ultrasonic wave, STC (sensitivity time control) has been conventionally used. The STC refers to signal processing of amplifying the detection signal of the ultrasonic echo while varying the amplification factor in accordance with the acquisition period. Here, the acquisition period refers to a period from a transmission time of the ultrasonic wave to a time point when the detection signal is acquired. That is, the deeper the region of the object where an ultrasonic wave is reflected and an ultrasonic echo is generated, the more largely the ultrasonic echo signal is amplified, and thereby, the detection signal having uniform intensity with respect to the depth direction of the object can be obtained.
Further, resolving power means discrimination capability of the object of imaging, and is represented by the minimum distance between two points as far as the two points can be discriminated. The resolving power includes axial resolving power with respect to the traveling direction (depth direction) of the ultrasonic beam and lateral resolving power with respect to the scanning direction of the ultrasonic beam. The axial resolving power included in the resolving power depends on the ultrasonic frequency and the sound speed.
On the other hand, the lateral resolving power is controlled generally in the following manner. As shown in FIG. 21, in the normal beam focus method, the region where the beam diameter is so small to have a focal depth “h”, i.e., the region where the lateral resolving power is good, is short. Therefore, the multi-stage focus method in which ultrasonic beams are synthesized while shifting the focal depth “h” by transmitting the ultrasonic beams in plural times while varying the focal position in the depth direction is performed. The multi-stage focus is described in detail in “Ultrasonic Wave Manual” (Ultrasonic Wave Manual Editorial Board, p. 440).
Furthermore, Japanese Patent Application Publication JP-2001-340338A discloses that filter processing is performed with respect to the acquired image signals by using different frequency filters in accordance with the distance from the ultrasonic transducer. For example, when the detection signal relating to the depth is amplified by STC, noise is also amplified together. If spatial filter processing is uniformly performed with respect to the obtained sound ray data in order to reduce such noise, the sound ray data in the high resolving power region becomes also blurred. In addition, as described above, since the beam diameter of the ultrasonic beam differs according to the depth, the lateral resolving power and the acoustic pressure intensity also differ according to the depth. In such case, by performing different filter processing on the sound ray data in accordance with the depth, variations in the response characteristics relating to the depth direction can be corrected.
FIGS. 22A and 22B show acoustic pressure intensity distributions (hereinafter, also referred to as “acoustic pressure intensity profiles”) formed on arbitrary focal planes within space by transmitting and receiving ultrasonic waves. These acoustic pressure intensity distributions are obtained by setting the following conditions in simulations. Here, the sound ray direction of the transmitted and received ultrasonic beam is represented by an angle θ and an angle φ. The angle θ is an angle relative to the first surface orthogonal to the transmission and reception surface of the ultrasonic transducer array, and the angle φ is an angle relative to the second surface orthogonal to the transmission and reception surface and the first surface.
FIG. 22A: sound ray direction θ=0°, φ=0°                focal length 70 mm        
FIG. 22B: sound ray direction θ=32.5°, φ=32.5°                focal length 70 mmFurther, constituent factors etc. of the ultrasonic transducer array are as follows, which are common in FIGS. 22A and 22B.        
Array constituent factors:                circular aperture two-dimensional array        0.35 mm×0.35 mm in element size        18.9 mm diameter in array size        (number of elements: 42)        
Transmission conditions:                number of used elements: 192        weighting with Gaussian distribution        Gaussian pulse 2.5 Hz, band 40%        
Transmission conditions:                number of used elements: 64        no weighting        
From the result of the simulations, the beam diameter of 2.6 mm in the case of FIG. 22A and the beam diameter of 3.2 mm in the case of FIG. 22B are obtained. In FIGS. 22A and 22B, the beam diameter represents the diameter of the acoustic pressure distribution on the surface where the acoustic pressure is under the peak by −6 dB.
As described above, it is seen that the beam diameter of the ultrasonic beam changes depending not only on the depth, but also on the sound ray direction of the ultrasonic beam. Accordingly, the acoustic pressure intensity and the lateral resolving power also differ depending on the sound ray direction. However, in the conventional signal processing course of the ultrasonic image, the adjustment of the response in accordance with the sound ray direction has not been performed.